Process for producing copper-clad base materials

ABSTRACT

In a process known per se for producing base materials copper-clad on at least one side, in which a winding core carrying a copper foil is wound with a reinforcing fiber, the laid fibers are impregnated with a solvent-free resin and the resin is cured fully to reach the C-stage, it is proposed to use, as the resin, a solvent-free epoxy-isocyanurate resin having a viscosity at processing temperature of less than 1000 mPa.s and an NCO content of more than 20%.

TECHNICAL FIELD

The invention relates to a process for producing copper-clad basematerials and to a process for producing multilayers from these basematerials.

Base materials (circuit boards) are boards copper-clad on one or bothsides, consisting generally of fiber-reinforced thermosetting resins,the resin being in the C-stage. They are used for the manufacture ofprinted circuit boards for the electronics industry and, if appropriate,for further processing into multilayers.

Multilayers are laminates consisting of a plurality of base materials.For producing them, the base materials (resin in the C-stage), providedwith appropriate conductive patterns, are pressed with interposed(adhesive) prepregs (resin in the B-stage) with application of pressureand heat, the resin of the prepregs curing fully to reach the C-stage.

Prepregs are resin-impregnated substrates, in general sheets of glassfiber fabric, the resin being in the not yet completely cured (B) state.

A conventional process for producing copper-clad base materials is thepressing of a plurality of plies of prepreg cut sheets with coveringplies of copper foil with application of pressure and heat, the resin ofthe prepregs being cured fully to reach the C-stage.

BACKGROUND ART

WO 88/01,938 has disclosed a process of the generic type for producingbase materials copper-clad on one or both sides by the so-calledfilament-winding technique and the use thereof for producingmultilayers. In this process, the winding mandrel used is a steel sheetcovered with a copper foil. This sheet is wound at the smallest possibleinclination with one ply of yarn filaments of a reinforcing material,preferably alkali-free glass, initially across the width, with thewinding mandrel rotating. After the whole surface of the sheet has beenwound, this winding step is repeated at an angle of 90°, whereby asecond ply is applied. The step of crosswise winding is repeated severaltimes until the desired glass content or the desired thickness of thewound plies has been reached. Instead of the conventional fabricreinforcement, a laid fiber structure is thus formed, in which theindividual filaments are not woven with one another but lie loosely ontop of one another.

After completion of the winding step, a closed mold is formed by meansof two metal sheets, covered on the inside with copper foil, and lateralsealing webs, which mold is evacuated to a residual pressure of lessthan 2 mm Hg. In an injection process or casting process, which is knownto those skilled in the art, for example, as the RTM process (resintransfer molding), a prepared and, if appropriate, heated resin mixtureis then allowed to flow in and, after the filling step has beencompleted, the resin mixture is allowed to cure fully at elevatedtemperature.

The casting resin mixture, described in WO 88/01,938, of epoxide resinand anhydride hardening agent has, however, a poor adhesion betweencopper and the plies, since a hardening agent must be added inrelatively large quantities, and this impairs the adhesive force (peelstrength).

In particular in the case of base material with a thin copper foil, forexample 17.5 /um thickness, whose use is indispensable for fine linetechnology and very fine line technology, the peel strength drops downto 1.0N/mm and lower, so that the requisite values are not reached.

OBJECT

It is therefore the object of the invention to provide a process of thegeneric type for producing base materials, which process leads to boardshaving higher peel strength of the copper foil and improved bonding ofthe plies.

A further objective of the invention is to provide an improved processfor producing multilayers, using base materials produced by thefilament-winding process (WO 88/01,938).

DISCLOSURE OF THE INVENTION

According to the invention, a process for producing base materialscopper-clad on at least one side, comprising (a) winding reinforcingfiber on a winding core carrying a copper foil, (b) impregnating thewound fibers with a solvent-free resin, and (c) curing the resin, theimprovement comprising employing a solvent-free epoxy-isocyanurate resinhaving a viscosity at processing temperature of less than 1000 mPa.saccording to DIN 53015, preferably less than 500 mPa.s, and an NCOcontent of more than 20%.

Surprisingly, it has been found that very high copper adhesion forces(peel strength)--typically between 1.6 and 1.8 N/mm--are achieved by theprocess according to the invention. The adhesion of the copper dependshere to a certain extent on the NCO content of the resin, higher NCOcontents leading to improved copper adhesion.

The epoxy-isocyanurate resin is preferably prepared from diphenylmethanediisocyanate, a bisphenol A-based epoxide resin (diglycidyl ether ofbisphenol A, epoxide number 0.585) and a suitable catalyst, for exampledimethylbenzylamine (EP-A2-0,272,563).

According to a first embodiment of the invention, after mixing, the saidcomponents are used directly for impregnating or soaking the laidfibers, corresponding to WO 88/01,838. The resin is then fully cured inan advantageously evacuated mold at a temperature of 100° C. to 150° C.(to reach the C-stage).

Advantageously, however, an at least partially (incipiently) reactedresin system of the abovementioned components is used, which can beadjusted to an appropriate viscosity and NCO content by addition of astopper solution according to EP-A2-0,272,563. Before the impregnationor drying of the laid fibers, an appropriate catalyst, for example asdescribed in EP-A2-0,272,563, must be added to the resin.

For preparing, according to the invention, the (adhesive) prepregsrequired for the multilayers, an epoxy-isocyanurate resin is likewiseused, but a resin having a higher viscosity and a lower NCO content. Theresin used for this purpose has already reacted to a relatively greatextent and is thus solid at room temperature. The NCO content is lessthan 17%, preferably less than 14%. The impregnation of the prepregsubstrates, preferably commercially available glass fabric webs ofalkali-free glass, is carried out either by means of a solution of theresin or by impregnation, preferably in vacuo, with a resin liquefied byheating. The solution or the heated resin contains in this case a latentcatalyst which, on later heating of the prepreg, starts the furtherreaction of the resin to reach the C-stage.

The resins used can be the epoxy-isocyanurate resins or solutionscorresponding to EP-A2-0,272,563, for example according to Example 2 orExample 3. The resin content of the prepregs is preferably 35-65% byweight, especially 40-50% by weight.

For producing the multilayers, printed circuits are first etched in amanner known per se out of the copper-clad base materials. These aresuperposed, each separated by an adhesive prepreg, and pressed in amanner known per se with application of pressure and heat to give themultilayers, the resin of the prepregs curing fully to reach theC-stage.

A particular advantage of the process according to the invention is thata resin of the same type can be used for the preparation of both thebase materials and the adhesive prepregs, even though the resin for thebase materials must be fully cured directly to reach the C-stage,whereas the resin for the prepregs must be kept in the B-stage forstorage stability. If appropriate, however, commercially availableprepregs based on epoxide resins can also be used as the prepregs.

BEST MODE FOR CARRYING OUT THE INVENTION

In an illustrative example for producing multilayers, a solvent-freepolyisocyanurate reactive resin based on diphenylmethane diisocyanateand a bisphenol A-based epoxide resin, having a viscosity of 300 mPa.sat 25° C. and an NCO content of 25% (mixture of in each case 50 parts byweight of the trial products Blendur^(R) I KU 3-4516 and Blendur^(R) IKU 3-4520 made by Bayer AG) is prepared and catalyzed with 2 parts byweight of an addition product of boron trichloride anddimethyloctylamine (made by Ciba Geigy). The mixture is introduced intoa mold which is preheated to 50° C. and which is wound crosswise with 4plies of glass yarn of ES 9-680 dtex on a sheet metal core, as describedin WO 88/01,838, and covered on the inner surfaces with a 35/um thickfoil of electrolyte copper. After curing for 2 hours at 200° C., theboard is released from the mold. The measured property data exceed thevalues set by DIN/IEC-249 for hard epoxide/glass fabrics of type FR-4,values of between 1.6 and 1.8 N/mm having been measured for the copperpeel strength.

The following were also determined: the glass transition temperature Tgwas 280° C.; the dimensional stability in both spatial directions was100-110 ppm in each case and the flammability rating according to UL-94was V1.

To prepare the adhesive prepregs, a polyisocyanurate resin solid at roomtemperature (Tg=40° C.) based on diphenylmethane diisocyanate and on abisphenol A-based epoxide resin, having an NCO content of 13%(commercial product Blendur I KU 3-4521, made by Bayer AG) is dissolvedin methyl ethyl ketone as the solvent. 2% of an addition product ofboron trichloride and dimethyloctylamine (made by Ciba Geigy) is addedas a catalyst to the solution. A glass fabric (US style 7628) is thenimpregnated with the 62% solution in a vertical impregnating machine ata maximum dry shaft temperature of 150° C. in such a way that a resincontent of 45% by weight is reached. The prepreg thus obtained, theresin of which has a residual content of 13% by weight of NCO groups, issealed airtight and moisture-tight in polyethylene film and is thusstorage-stable.

From the board obtained in the first process step, a two-sided circuitis produced by the conventional print-and-etch process, this issubjected to the likewise conventional black oxidation process withcommercially available oxidizers, 2 sheets of the prepreg obtained inthe second process step and 1 sheet of 35 /um thick foil of electrolytecopper are then laid around each of the two sides and, finally, thewhole is laminated in a press for 2 hours at 180° C. between 2 steelsheets and conventional press pads. After heat treatment of the finishedboard for 2 hours at 200° C., it is further processed in theconventional manner to give a 4-ply multilayer.

We claim:
 1. In a process for producing base materials copper-clad on atleast one side, comprising:(a) winding reinforcing fibers on a windingcore carrying a copper foil, (b) impregnating the resultant wound fiberswith a solvent-free resin, and (c) curing the resin fully, theimprovement comprising employing as the resin a solvent-freeepoxy-isocyanurate resin having a viscosity at processing temperature ofless than 1000 mPa.s and an NCO content of more than 20%.
 2. A processaccording to claim 1, wherein the resin is an epoxy-isocyanurate resinproduced from diphenylmethane diisocyanate and a bisphenol A epoxideresin and a catalyst.
 3. In a process for producing multilayers,comprising compacting a plurality of base materials with interposedprepegs with application of pressure and heat, the improvementcomprising producing said base materials by a process comprising windingreinforcing fiber on a winding core carrying a copper foil, impregnatingthe resultant wound fibers with a solvent-free epoxy-isocyanurate resinhaving a viscosity at processing temperature of less than 1000 mPa.s andan NCO content of more than 20% and wherein the resin for said prepegsis an epoxy-isocyanurate resin in the A state or B-stage, which is solidat room temperature and has an NCO content of less than 17%.
 4. Acircuit board having a copper foil surface reinforced by glass fibersimpregnated with an epoxy-isocyanurate resin having an NCO content ofmore than 20%.
 5. A circuit board according to claim 4, wherein saidcopper foil has a peel strength of 1.6-1.8 N/mm.
 6. A multilayercomprising a plurality of layers having a copper foil surface reinforcedby glass fibers impregnated with an epoxy-isocyanurate resin having anNCO content of more than 20% , said layers being joined by anepoxy-isocyanurate resin prepeg having an NCO content of less than 17%.7. An article produced by the process of claim
 1. 8. An article producedby the process of claim
 2. 9. An article produced by the process ofclaim
 3. 10. A process according to claim 1, wherein the copper foil hasa thickness of not more than 35 μm.
 11. A process according to claim 3,wherein the copper foil has a thickness of not more than 35 μm.
 12. Acircuit board according to claim 5, wherein the copper foil has athickness of not more than 35 μm.
 13. A multilayer according to claim 6,wherein the copper foil has a thickness of not more than 35 μm.
 14. Anarticle according to claim 7, wherein the copper foil has a thickness ofnot more than 35 μm.
 15. An article according to claim 8, wherein thecopper foil has a thickness of not more than 35 μm.
 16. An articleaccording to claim 9, wherein the copper foil has a thickness of notmore than 35 μm.